1. Field of the Invention
The invention relates to a temperature probe for large surfaces, comprising at least two elongated conductors, space from each other and connected through a semi-conducting substance, such substance acting as a sensor, and said sensor perhaps closely shielded by a tube-like metallic sleeve.
2. Description of the Related Art
Temperature sensors are, for example, known from DE-B-17 73 577 or EP-B-0 078 675. The temperature sensor may thus consist of a long nickel wire coaxially surrounded by a nickel tube. To provide connection between the connecting materials, the tube is filled with a semi-conducting substance consisting of nickel oxide. Temperatures are sensed by the electrical resistance between the nickel conductors, said resistance substantially being determined by the semi-conducting substance in place between the conductors. Even at a locally narrowly circumscribed and relatively small temperature rise ("hot spot"), the electrical resistance between the conductors, determined by the high negative temperature coefficient of the electrical resistance of nickel oxide, drops strongly and can be recorded by a measuring device attached at the ends of the conductors. To permit measurement of said electrical resistance, the inner conductor is passed to the outside through a gas-tight, electrically insulating feedthrough. The latter is connected, via a takeup tube, with the protecting tube of the sensor. Nickel oxide being strongly hygroscopic, care must be taken to see that moisture cannot penetrate. Otherwise, hot spots would be simulated which actually are not there, namely where said penetrated moisture produces a short circuit between the electrical conductors. In this case, the temperature sensor would cease to work, with the result that temperature changes can no longer be recorded.
A known procedure is to provide protection against penetrated moisture by closely surrounding the outer nickel tube with a protective sheath which may consist of high-grade steel or Inconel. Such a sheath may be formed by cold-welding to the nickel tube. As similar temperature sensors are often placed on the monitored surface, such as a high-pressure temperature reactor, in a serpentine pattern, the risk still is that said sheath develops needle-like cracks and becomes leaky.
In another known temperature sensors two wires of different materials forming a thermocouple are placed without contact in a protecting tube, the tube itself is filled with a semiconducting substance. The two wires act as a thermocouple as soon as the electrical resistance of the semiconductor, following a rise in temperature, breaks down at one location and the two wires short-circuit at this location to form a measuring point. However, thermocouple arrangements of this type have the disadvantage that when a short-circuit happens through penetrated moisture it cannot be determined whether an unacceptable temperature rise has occurred, or whether the signal was prompted by penetrating liquid.